Abnormalities in RNA processing and translation within neurons likely contribute to Autism Spectrum Disorders (ASD). For example, mutations in Fragile X Mental Retardation protein, an RNA binding protein involved in RNA trafficking and translational regulation at the synapse, represent the most common single gene cause of ASD. More recently, human genetic studies identified the RNA binding protein Rbfox1 (also known as A2BP1) as another candidate autism gene. Rbfox1 binds a well-defined RNA sequence, (U)GCAUG, and functions in the nucleus as a regulator of RNA splicing. Rbfox1 itself is alternatively spliced into nuclear and cytoplasmic forms. We show that cytoplasmic Rbfox1 localizes to dendrites and synapses in mouse hippocampal neurons. Many neuronal RNAs contain conserved (U)GCAUG stretches in their 3' untranslated regions (3'UTRs), and our data indicate that cytoplasmic Rbfox1 regulates the stability and/or translation of these mRNAs. In addition, our experiments suggest that Rbfox1 regulates translation by interfering with microRNA (miRNA)-mediated translational repression of some target mRNAs. Many of the mRNA targets of cytoplasmic Rbfox1 have been identified as targets of Rbfox1 in a module of genes that are down regulated in brains of autistic subjects. We propose that dysregulation of mRNA stability and translation in neurons is an important component of the pathophysiology of ASD. Our proposal is aimed at 1) identifying the cytoplasmically localized mRNA targets of Rbfox1 and at 2) determining the mechanisms whereby Rbfox1 regulates their stability and/or translation. The results of our proposed studies may reveal fundamental cell biological mechanisms and specific molecular targets that underlie neural circuit dysfunction in neurodevelopmental disorders, including Autism Spectrum Disorders.